The present invention relates to a technique for using foils to shape ion beams.
Ion beams have many important uses in scientific research, medicine, and industrial applications. The uses include, but are not limited to, research in fundamental particle physics, research in nuclear physics and chemical, isotope generation, medical research and treatment, imaging, writing on hard materials, cutting, etc. Generating, shaping and directing ion beams requires equipment including ion generators, magnetic field generators, and magnetic field lenses, as well as complex circuitry to control their performance. Such equipment is complex and expensive.
Ion beams by their very nature, are composed of charged particles. The charging of the particles is necessary to enable the acceleration of the particles forming the beam. Directing charged particle beams requires complex and expensive equipment because the charged particles tend to repel each other. Therefore, controlling an ion beam requires further complex and expensive equipment.
Ion beam generators, generally, have a main beam that is directed onto a target. Van De Graff tandem generators are typically used to generate low energy ion beams. Cyclotron accelerators are typically used to generate high-energy ion beams.
In applications using ion beams, one typically desires to maintain the integrity of the irradiated targetxe2x80x94unless, of course, an application specifically is designed to destroy or change the irradiated target. Economic and efficiency considerations require that one attempt to use as much of the power of an ion beam as possible. Ideally, one would prefer to direct all of the power in a generated ion beam onto a target. The intensity profiles of ion beams, however, have high intensity regions (hot spots). For example, the cross section of an actual ion beam is well approximated by the Gaussian distribution, with an intensity peak at the center. The temperature distribution on a target is determined by the intensity distribution of the incident power: regions in a target exposed to higher power intensity have higher temperatures. Hot spots, therefore, act as seeds for starting the thermal damage of targets and, thus, limit the efficiency of using the total power available in the ion beam.
Moreover, not all target materials are in solid form. For example, many applications require, or use, targets having gaseous or liquid form. Such targets require containerxe2x80x94usually a thin foilxe2x80x94to contain the target material. However, container walls absorb some of the ion beam irradiated onto the target and, thus, also heat up. Non-uniform intensity profiles of irradiated ion beams, therefore, cause loss of target material containment by rupturing container walls (due to thermal damage) at points exposed to the hot spots of the incident ion beam.
Furthermore, a new generation of cyclotrons have increasing power capability, which make them even more useful in isotope generation. However, as explained above, targets lag behind in their ability to handle the higher power of ion beams generated by the new cyclotron resonators. Optimizing the design of targets, using new alloys as target substrates, and enhancing cooling efficiency would allow targets to handle ion beams having higher powers. Such improvements, however, are reaching the limits of their possible refinements.
In addition to thermal damage, hot spots lead to non-uniform products. For example, many applications require special materials composed from isotopes that are generated by irradiating ion beams onto a parent target. Therefore, ion beams having hot spots lead to the non-uniform distribution of isotopes within the target material and therefore lower the yield of isotope generation and parent material utilization.
To increase the efficiency of using the power available in an ion beam, therefore, users must reshape the intensity profile of the ion beam by removing ions from hot spots to lower intensity regions within the cross section of the ion beam. Ideally, it is desirable that an ion beam be obtained that has a top-hat intensity profile so that all of the power can be usedxe2x80x94a desire that is practically impossible to satisfy.
One way to reduce the intensity of hot spots in a beam is to defocus the beam and trim it to the target shape. The defocusing reduces the peak energy deposited onto the target by shifting it to the wings and, thus, reduces the highest temperature of the target surface. However, such trimming wastes portions of the generated energy beam and further increases the ambient radiation levels during operation. This is an inefficient and unsafe result. In normal practice, only about 10% to 20% of the beam is typically trimmed.
Another way to reduce the peak intensity is to use sophisticated multiple-pole magnetic lenses (e.g., specially designed new configurations for sexapole magnetic lenses) to reshape and flatten the beam cross section. The drawback in implementing such an approach is the design and manufacturing cost of such complex magnetic lenses combined with their relative invariant nature and extra floor space needed regarding placement. Currently, such approaches, therefore, have limited practical use.
Similar arguments restrict the use of rotating or swept beams. In addition, such sweeping beams still have high intensity density and, therefore, cause instantaneous stresses in an irradaited target. The thermal cycling of these stresses lead to the premature failure of the irradiated target as a result of metal fatigue. Thus, a need exists for a way to increase the use of available power in an ion beam.
The invention presents an approach that uses plural separated foils to shape an ion beam so that the intensity density of hot spots in the ion beam can be lowered. More particularly, plural foils are placed in close proximity to each other, wherein at least one foil intercepts a portion of the beam to strip electrical charge from ions in different portions of the beam at different times and, thus, shape the ion beam. At a basic level, the inventive approach places plural foils so that the distance between planes of successive foils is preferably a small fraction of the radius of curvature of the beam""s cyclotron orbit.
The inventive approach has an advantage of using low cost implements, of a very simple and controllable nature, to shape the intensity density of ion beams generated by existing accelerators and enhance their utility. Moreover, it shapes the intensity within an ion beam without sacrificing energy from the ion beam. The inventive approach, in a simple and inexpensive manner, can be used to divide a single ion beam into plural ion beams that are nearly parallel and that have a controllable separation. As such, a single ion beam can be divided into plural beams so that the highest intensity density on an irradiated target can be lowered, with the total energy deposition onto a target not being reduced.